The present application is based on Japanese Patent Application No. 10-294999 filed Oct. 16, 1998, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to an ocular lens such as a contact lens placed on an eyeball or an intraocular lens inserted within an eye. More particularly, the invention is concerned with a multifocal ocular lens having a plurality of vision correction regions with respective different values of optical or vision correction power.
2. Discussion of the Related Art
As an ocular lens used for vision correction of an eye suffering from deteriorated accommodation such as presbyopia, there has been proposed a multifocal ocular lens having a plurality of vision correction regions which provide respective different values of optical power. As one example of the multifocal ocular lens, JP-A-63-95415 and JP-A-1-319729 disclose an alternating or translating vision type contact lens, wherein the vision correction regions with different values of optical power are selectively and alternatively used as needed in observing objects, with a shift of the visual axis of the lens user or wearer. As another example of the multifocal ocular lens, U.S. Pat. No. 4,580,882 and JP-A-2-217818 disclose a simultaneous vision type contact lens, wherein the vision correction regions are simultaneously used, and one of the vision correction regions through which a desired image is observed is selected by an action of the lens wearer""s brain.
In either of the alternating and simultaneous vision type ocular lenses described above, there is proposed: a so-called bifocal ocular lens having two vision correction regions, i.e., a near vision correction region through which near objects are observed and a distant vision correction region through which distant objects are observed; and a multifocal ocular lens having such near and distant vision correction regions and at least one intermediate vision correction region which is interposed between those near and distant vision correction regions and which provides an optical power different from the optical powers of the near and distant vision correction regions. The multifocal ocular lens has at least three vision correction regions having respective different optical power values.
The bifocal ocular lens having the two vision correction regions, i.e., the near and distant vision correction regions, tends to suffer from a jump or overlapping of the image of an object which is located intermediate between near and distant objects. This is because the bifocal ocular lens has only two focal points which correspond to the respective near and distant vision correction regions. Further, the bifocal ocular lens has surface discontinuity at a junction of the near and distant vision correction regions, in other words, the two vision correction regions are not smoothly connected to each other at the junction therebetween, deteriorating a wearing comfort as felt by the lens user.
In the multifocal ocular lens, each of the plurality of vision correction regions inevitably has a relatively small radial dimension, so that the optical power of the lens changes in steps in its radial direction, corresponding to the plurality of vision correction regions. In this case, the near and distant vision correction regions do not have sufficiently large surface areas, whereby the images to be obtained through these near and distant vision correction regions undesirably tend to be blurred. Moreover, since the optical power in the at least one intermediate vision correction region changes in steps, the multifocal ocular lens does not assure a high degree of visual acuity of the object located intermediate between the near and distant objects, so that the multifocal ocular lens is likely to suffer from a problem of ghosting or double imaging.
JP-A-5-181096 discloses a multifocal ocular lens having a near vision correction region, an intermediate vision correction region, and a distant vision correction region, so that the optical power of the multifocal ocular lens continuously changes in its radial direction from the value of one of the near and distant vision correction regions to the value of the other of the near and distant vision correction regions. This multifocal ocular lens was developed in an attempt to improve its wearing comfort as felt by the lens user, and assure clear viewing of the objects located intermediate between the near and distant objects. However, the proposed multifocal ocular lens is still unsatisfactory in providing sufficiently clear viewing of both of the near and distant objects.
JP-A-9-26559 proposes a bifocal ocular lens having a plurality of pairs of the near and distant vision correction regions which are arranged such that the near and distant vision correction regions are alternatively located in the radial direction of the ocular lens. Like the ordinary bifocal ocular lens having one pair of near and distant vision correction regions, the proposed bifocal ocular lens does not assure clear viewing of the object located intermediate between the near and distant objects, and suffers from the jump or overlapping of the image.
It is therefore an object of the present invention to provide a multifocal ocular lens having a novel structure, which multifocal ocular lens is capable of assuring sufficiently enhanced visual acuity of an object located intermediate between near and distant objects with high stability, while at the same time assuring a high degree of visual acuity of both of the near and distant objects.
The above object of the present invention may be attained according to the principle of the invention which provides a multifocal ocular lens having a vision correction area including at least one distant vision correction region having a first optical power value for distant vision correction and at least one near vision correction region having a second optical power value for near vision correction, wherein the vision correction area further includes a central intermediate-distance vision correction region which has a third optical power value for intermediate vision correction between the first and second optical power values and which is located in a central portion of the vision correction area such that the distant and near vision correction regions are located radially outwardly of the central intermediate-distance vision correction region.
The multifocal ocular lens of the present invention constructed as described above assures a high degree of visual acuity of the objects at desired distances, i.e., at the desired far and near distances, owing to the first and second optical power values (distant and near vision correction power values) given to the distant and near vision correction regions, respectively. Further, the present multifocal ocular lens is capable of assuring a high degree of visual acuity of objects located at an intermediate distance between the far and near distances, owing to the central intermediate-distance vision correction region which has the third or intermediate vision correction optical power value intermediate between the first and second or distant and near vision correction power values. For assuring sufficiently high visual acuity at both of the far and near distances, the distant and near vision correction power values of the distant and near vision correction regions are preferably made constant in the radial direction of the ocular lens. However, the distant and near vision correction power values may be slightly changed in the radial direction for the purpose of reducing aberration.
Since the central intermediate-distance vision correction region is located in the central portion of the vision correction area in the present multifocal ocular lens, the intermediate-distance vision correction region can be advantageously positioned on the pupil of the lens user""s eye, irrespective of circumferential positioning stability of the lens, displacement of the lens during its use on the eye, and a change in the diameter of the pupil. Accordingly, the present multifocal ocular lens is capable of assuring a high degree of visual acuity of the object located at the intermediate distance between the near and far distances, with high viewing stability.
In the present multifocal ocular lens, the optical power of the central intermediate-distance vision correction region is determined so as to be held within a range between the two constant optical power values of the distant and near vision correction regions, to thereby assure enhanced visual acuity of the object located at the intermediate distance between the near and far distances.
The principle of the present invention is applicable to various kinds of ocular lenses such as contact lenses and intraocular lenses, particularly to the contact lenses, which can be advantageously used as presbyopia correction contact lenses. Further, the present invention is applicable to both of soft and hard contact lenses.